This invention relates generally to medical bone fixation/stabilization systems which are devices that commonly comprise a system of screws, plates and rods that are attached to bones in various surgical procedures. The invention more particularly relates to an improved screw for use in such bone fixation/stabilization systems. The improved screw has structural features that guide the screw along the trajectory of a bore or passageway that has been preformed by a surgeon. The self guiding feature allows the surgery to be performed with a smaller incision that is characteristic of surgical procedures that use a guide wire while allowing the guide wire to be removed prior to insertion of the screw so that complications that sometimes occur when a screw is guided by a guide wire can be avoided.
A common use of bone fixation/stabilization systems is for spinal fixation and stabilization. For this purpose, a spinal fixation system is attached to two or more adjacent vertebrae using pedicle screws to mechanically the attach rods and/or plates to the vertebral bone.
In one prior art spinal fixation procedure, after making an initial incision, the surgeon uses a drill to form a bore in the vertebral bone, then uses a tap to make a female threaded bore and then inserts the screw into the bore. A slight variation eliminates the tap and instead uses a self tapping screw. A disadvantage of this procedure is that a relatively larger incision is made and the soft tissues are distracted to provide access to and a view of the vertebrae.
Another prior art procedure can be performed through a considerably smaller incision. After the initial incision, a guide wire, referred to as a k-wire, is inserted with the assistance of fluoroscopy to position the wire through vertebral bone along a trajectory selected by the surgeon as the desired eventual trajectory of the screw. Fluoroscopy is needed because the small size of the incision impairs the view of the surgeon. The properly positioned k-wire is then used as a guide for guiding a drill, tap and pedicle screw. The drill, tap and the pedicle screw are all cannulated so that they can follow the trajectory of the wire that extends through their central, longitudinal passageway. In this procedure, after the k-wire is positioned, the exposed k-wire is threaded through a drill and/or tap which is then used to form a bore in the vertebral bone. Then the drill and/or tap is removed but the k-wire remains in place. A cannulated pedicle screw, guided by the k-wire along the selected trajectory followed by the wire, is then inserted through the soft tissue and into the bore formed in the vertebral bone. After installation of the pedicle screw, the wire is pulled out. Consequently, once the surgeon has properly positioned the k-wire, the k-wire guides the drill and/or tap and also the pedicle screw along the trajectory of the k-wire, despite the poor visibility available to the surgeon because of the relatively small size of the incision.
One problem that sometime occurs with this k-wire procedure is that before the pedicle screw is installed, the k-wire may be unintentionally dislocated or partially withdrawn. In that event, the surgeon must relocate it. Another problem that sometimes occurs is that, as the pedicle screw is moving along the k-wire during installation, a bend or kink may develop along the k-wire ahead of the direction of insertion of the pedicle screw. Such a bend or kink can catch on the advancing inner end of, and stop further insertion of, the pedicle screw. An even greater problem occurs if the pedicle screw is advanced after it catches on a bend or kink in the k-wire and thereby forces the k-wire through the anterior cortex of the vertebral body and into the aorta and vena cava causing serious medical complications. Consequently, if the pedicle screw becomes jammed against a kink or bend in the k-wire, the screw and the k-wire must be removed and replaced before placement of the screw.
Therefore, it is an object and feature of the invention to provide a surgical bone fixation screw that can be used in a procedure that has the advantage of requiring only the smaller incision that is possible with the k-wire procedure, but is self guiding so that the k-wire can be removed before insertion of the pedicle screw thereby avoiding the above described problems and complications. A bone fixation screw according to the invention provides the new result that the screw is guided by an elongated guide end segment on the distal end of the screw itself, making guidance by a k-wire unnecessary.